It is common practice by injection moulding of thermoplastics or like material to provide the discharge orifice to the dies with needle-valve to avoid formation of a sprue on the article to be produced. The needle-valves are arranged to close the discharge orifice by the end of the injection and by starting the injection the needle will automaticly be retracted from the discharge orifice by the pressure of the molten material against the action of spring means which will return the needle to the closing position when the pressure ceases.
By dies for great articles and especially dies for the producing of a great number of small articles the die will be provided with a plurality of discharge orifices and the molten material which under high pressure is delivered from the injection moulding machine is delivered to orifices through a hot runner panel which is a rather solid element with distribution channels and heating elements to ensure the proper temperature of this component.
For adjusting or removing the needle-valves it has generally been necessary to disassemble the dies and the hot runner panel and it has also been difficult to obtain adequate sealing against the high pressure by which the molten material is introduced in the dies, so that it could not be avoided that a small part of the molten material seeps out and solidifies on the outer surface of the dies which may necessitate a frequent and cumbersome cleaning. Further the tip of the needle generally is arranged for in the closed position to cooperate with a frusto-conical seat in the discharge orifice, so that when molten material solidifies on this seat it would not be possible to maintain the desired closing position of the needle.